Sure. These plans have a $149 early upgrade option after 12 months. You pay that and pick any phone you like. Return the old phone (in good condition). You start a new contract again.
Note doing this essentially forfeits the 12-24 months of payments you made paying off the phone you have. Possibly a better option is to just buy the next model you want outright. Sell your iPhone 11 on eBay for a good price. Put the SIM from the old phone in your new phone and continue on.
That way you don't get slugged $149. You don't forfeit the payments you've already made to Optus for the old phone. You have a new model. In 12-24 months you will have paid off the old phone 100% and also own your new phone 100%. Do some counting of the tco (i.e. how much does any choice cost you in total over the next 36 months.)
Please let us know the specific plan you're on, Ikey28 and we'll be happy to confirm 🙂
Should also be in the CIS as mentioned by the others.